The present invention is related to a method for protecting a specific region applied in preparing a specimen, and more particularly to a method for protecting a specific region in a sample applied in preparing an ultra thin specimen to be observed by a microscope.
Nowadays, due to the improvement in the manufacturing process of the microelectronic element and tendency of demanding a smaller line width, the semiconductor analysis is getting more and more difficult, particularly in the analysis technology of the finished dynamic random access memory (DRAM) having a size less than 0.25 xcexcm.
In the semiconductor industry, it is common to use a scanning electron microscope (SEM) to observe the surface condition of a wafer and use a transmission electron microscope (TEM) to examine the microstructure of a wafer to ensure that the fabricated microelectronic elements satisfy an expected standard. However, because the resolution of the scanning electron microscope (SEM) is not good enough to observe the detailed structure of the microelectronic elements, it is replaced by the transmission electron microscope (TEM) to make major failure analysis. As one may realize, most of the problems in TEM application are related to sample preparation, which is the most difficult part of TEM analysis. Traditionally, a sample is cut from a wafer to be examined. After thoroughly polished, the sample becomes ultra-thin and is ready to be examined by the transmission electron microscope (TEM) for determining the quality of the wafer. However, during the preparing process, an optical microscope and a laser mark must be employed so that an ultra-thin specimen for fixed-point failure analysis can be prepared easily.
At the present time, a conventional method for marking a specific region in a sample utilizes a laser technique. However, there are some defects by utilizing the laser technique to make the marks.
(1) The laser cannot be focused further so that the width of the mark made by the laser technique is many times than 0.25 xcexcm. In addition, due to the resolution limitation of the optical microscope, a single-bit failure analysis which has a size less than 0.35 xcexcm cannot be carried out by employing the laser technique to prepare the specimen.
(2) The laser technique utilizes a laser light beam having a high energy to remove the material in a mark region of the sample so as to form a concavity to be observed by microscopy. However, due to the damage of the high-energy laser light beam, a plurality of cracks are formed on the periphery of the concavity. When the sample is milled and/or polished to make an ultra-thin TEM specimen, those cracks may be propagated due to the local stress to form additional defects in the TEM specimen, which may seriously influence the analysis result of the microelectronic element.
A focus ion beam technique (FIB) was designed to make TEM sample instead of the laser technique. The focus ion beam (FIB) technique utilizes focused high-energy gallium ions to remove materials from both sides of the desired region. The focus ion beam (FIB) technique can offer a reliable method to precisely obtain a cross section of the specific area, and the sample can be thinned to less than 0.1 xcexcm. However, this method requires very expensive equipment and cost for preparing an ultra-thin specimen, which further limits the usage of this method. Therefore, it is not worthy to prepare an ultra-thin specimen directly by the focus ion beam (FIB) technique.
For the above reason, it is desirable to develop a low-cost and effective method for protecting a specific region in the sample applied in preparing an ultra-thin specimen of 0.25 xcexcm or less size dynamic random access memory (DRAM).
It is therefore a primary object of the present invention to provide an effective method for protecting a specific region in a sample applied in prepared an ultra-thin specimen to be observed by a microscope.
Another object of the present invention is to provide a protecting device formed on the sample for protecting a specific region to be milled and polished during the preparing process so that the sample can be polished to a required thickness without any damage.
It is further an object of the present invention to provide an effective method for protecting a specific region in a sample so as to avoid necessary damage for the ultra-thin specimen and reduce the preparing cost for the ultra-thin specimen.
In order to accomplish the object of the present invention, a method for protecting a specific region in a sample applied in preparing an ultra-thin specimen is provided. The method includes the steps of (a) forming a first concavity on a first side of the specific region by a focus ion beam (FIB) technique, (b) forming a second concavity on a second side of the specific region opposite to the first side by focus ion beam technique, (c) filling the first concavity and the second concavity with a first metallic packing and a second metallic packing respectively, and (d) forming a third metallic packing over the specific region, the first metallic packing and the second metallic packing to define a protecting device for protecting the specific region.
In accordance with one aspect of the present invention, the third metallic packing has a first end connecting with the first metallic packing and a second end connecting with the second metallic packing to form the protecting device.
Preferably, the third metallic packing has a first end protruding beyond the first metallic packing and a second end connecting with the second metallic packing to form the protecting device. More preferably, the third metallic packing has a first end protruding beyond the first metallic packing and a second end protruding beyond the second metallic packing to form the protecting device.
In accordance with another aspect of the present invention, after the step (c), the method further includes a step of forming a third concavity on the first side of the specific region adjacent to the first concavity and then filling the third concavity with a fourth metallic packing. Preferably, the third metallic packing is extending to connect with the first, the second and the fourth metallic packing to form the protecting device.
Preferably, after the step (d), the method further includes a step of (d1) forming a fourth concavity on a third side of the specific region. More preferably, after the step (d1), the method further includes a step of (d2) forming a fifth concavity on a fourth side of the specific region opposite to the third side.
In accordance with another aspect of the present invention, after the step (d), the method further includes a step of (e) polishing one side of the sample with a higher polishing rate till the protecting device is partially polished.
In accordance with another aspect of the present invention, after the step (e), the method further includes a step of (f) polishing said side of the sample with a lower polishing rate till the protecting device is completely exposed.
In accordance with another aspect of the present invention, after the step (f), the method further includes a step of (g) stopping polishing said one side of the sample and then polishing the other side of the sample opposite to said one side.
Preferably, the step (c) is executed by a sputtering technique.
Preferably, the microscope is one selected from the group consisting of an optical microscope (OM), a transmission electron microscope (TEM), and a scanning electron microscope (SEM)
Preferably, each of the concavities is substantially a parallelepiped having a length and width ranged from 1 to 1.5 xcexcm and a depth ranged from 2 to 3 xcexcm.
Preferably, each of the metallic packings is made of one selected from Platinum (Pt) and Tungsten (W).
Preferably, the distance between the first concavity and the second concavity is about 5 xcexcm.
Preferably, the ultra-thin specimen has a thickness of about 0.2 xcexcm.
Preferably, the specific region has a length of about 5 xcexcm and a width of about 0.2 xcexcm.
It is further another object of the present invention to provide a method for protecting a specific region in a sample. The method is applied in preparing an ultra-thin specimen to be observed by a microscope. The method include the steps of (a) forming a first concavity on a first side of the specific region by a focus ion beam (FIB) technique, (b) forming a second concavity on a second side of the specific region opposite to the first side by the focus ion beam technique, (c) filling the first concavity and the second concavity with a first metallic packing and a second metallic packing respectively, and (d) forming a third metallic packing on the specific region, wherein the third metallic packing has a first end surface connecting with an inner surface of the first metallic packing and a second end surface connecting with an inner surface of the second metallic packing to define a protecting device for protecting the specific region.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: